1. Field of the Invention
The present invention relates to flame holders disposed in an afterburner section of a gas turbine engine, and particularly to an improved close-coupled mount for securing the flame holder in the afterburner section of the engine.
2. Description of the Related Art
High performance jet aircraft engines often include reheat thrust augmentation as a means of substantially increasing the thrust produced by the engine for brief periods. This thrust augmentation is accomplished by injecting fuel into an after burner section, downstream of the turbine, and igniting the fuel in a combustion zone in the afterburner section to increase the exhaust gas temperature. This increase in temperature results in an increase in thrust as the hot products of combustion expand through the jet nozzle. In order to stabilize the combustion zone in the afterburner section, turbulence producing flame holders are positioned downstream of the fuel injectors.
A typical jet aircraft engine configuration is shown schematically in FIG. 1, and is referred to generally as 10. It includes a turbine engine section 12 generally defined by arrows A, and an afterburner section 14 generally defined by arrows B. The exhaust gas flow path through engine 10 is represented by arrows C. Afterburner section 14 is located downstream from turbine engine section 12 in the exhaust gas flow path. Afterburner section 14 typically includes a plurality of fuel injectors 16 extending radially inward from an outer casing 18 of afterburner section 14. Fuel injectors 16 are typically surrounded by fairings 20 which form heat shields about the fuel injectors. Flame holders 22 are positioned downstream of fuel injectors 16 in afterburner section 14 and are typically fastened to fuel injectors 20 by brackets 24.
In thrust augmentor or afterburner construction, it is desirable to control the combustion zone in the afterburner section. To this end exhaust gases and fuel pass over flame holders 22 and a turbulent wake which defines the combustion zone is created downstream from the flame holders. The fuel is vaporized by the hot gas stream exiting from turbine engine section 12, and the turbulent wake behind flame holders 22 creates a combustible mixture of exhaust gases and fuel for efficient burning.
In modern thrust augmentation exhaust gas temperatures are high enough that special precautions must be taken to avoid undesirable auto ignition of the fuel upstream of the flame holder at the fuel injectors. A key anti-auto ignition precaution involves injecting the fuel immediately upstream of the flame holder so that the fuel is not vaporized and mixed until it reaches the combustion zone in the turbulent wake behind the flame holder. This configuration is often referred to as close-coupled fuel injection, to which the present invention is directed.
In addition to close-coupled fuel injection, it is important that turbulence upstream of the flame holder be strictly limited or eliminated altogether since such turbulence produces conditions, i.e. sufficient mixing of the fuel and hot products of combustion, which tend to promote auto ignitition upstream of the combustion zone.
There are at least two additional drawbacks associated with prior art close-coupled flame holder attachments which have been hampering advanced thrust augmentor design during recent years. These additional problems relate to manufacture and maintenance of the flame holder.
With the emphasis strongly on maintainability in new jet engine designs, it is highly desirable, if not mandatory, that the flame holder be easily replaceable on the flight line by a mechanic entering the exhaust duct of the jet engine. Prior art attempts at providing close-coupled mounting of the flame holders to the fuel injectors have not optimized accessibility of flame holder attachments for easy maintenance while simultaneously providing sufficient heat resistance and turbulence reduction in the afterburner section.
Exhaust gas temperatures in modern jet engines have increased to the point that metallic flame holders have questionable life expectancy. To enhance the life expectancy of the flame holders ceramic technologies are being investigated and pursued in the search for an improved flame holder material. The design of such ceramic flame holders, however, has been hindered for many years by the manufacturing and brittleness limitations of ceramic.
Various proposals have been made for providing a flame holder mount which satisfies the needs of an advanced high temperature turbo jet engine. FIGS. 2A and 2B show some of the typical flame holder mounting configurations which have been utilized. Each of these prior art flame holder mount configurations utilize a bracket of various configurations such as 26 and 28 fastened to a fairing 20 which support flame holder 22. The outward projection of the brackets 26 and 28 from fairing 20 creates exhaust gas flow blockages and promotes turbulence upstream of flame holder 22. Such turbulence is undesirable since it is likely to cause local burning upstream of the combustion zone. The need to provide clearance in the attachment joints for fit-up of the flame holder with the supporting bracket and fairing adds directly to the aforesaid flow blockage and turbulence promotion and, thus, further increases the auto ignition risk.
As shown in FIG. 2A, pin 30 attaches bracket 26 to fairing 20; and in FIG. 2B, pin 32 attaches flame holder 22 to bracket 28, and pin 34 attaches bracket 28 to fairing 20. Each pin 30 and 34 projects outwardly with respect to fairing 20 thus causing undesirable flow blockages in the exhaust gas flow path upstream of the combustion zone. Moreover, pins 30, 32 and 34 are positioned upstream of flame holder 22 such that access from the rear or apex of the flame holder ring, as required for removal or installation of the flame holder, is difficult. Therefore, such designs require considerable maintenance man hours and/or special tools or fixtures for servicing.
As noted above, as far as is known, all prior art attempts for providing a close-coupled mounting structure for flame holder 22 include some type of bracket projecting from fairing 20. Such projections or brackets are difficult and costly to produce in the kind of ceramic and carbon composite materials that will tolerate the severe temperature and pressure conditions in advanced jet engines.